Method and apparatus for receiving a timing advance command in a wireless communication system

ABSTRACT

The present invention provides a method of receiving a timing advance command by a user equipment in a wireless communication system. A terminal receives information on a time advance group from a base station, and also receives the tuning advance command corresponding to the time advance group from the base station.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/826,158, filed on Aug. 13, 2015, which is a continuation ofU.S. patent application Ser. No. 13/520,207, filed on Jul. 2, 2012, theentire disclosure of each of which is hereby incorporated by referencefor all purposes as if fully set forth herein. U.S. patent applicationSer. No. 13/520,207 is a U.S. National Stage Entry of PCT InternationalApplication No. PCT/KR2011/000111, filed on Jan. 7, 2011, and claims thebenefit of Korean Patent Application No. 10-2011-0001072, filed on Jan.5, 2011; and claims the benefit of U.S. Provisional Application No.61/293,185, filed on Jan. 7, 2010.

TECHNICAL FIELD

The present invention relates to wireless communications, and moreparticularly, to a method of receiving a timing advance command and amethod of transmitting the timing alignment command in a wirelesscommunication system.

BACKGROUND ART

3rd generation partnership project (3GPP) long term evolution (LTE) isan improved version of a universal mobile telecommunication system(UMTS) and is introduced as the 3GPP release 8. The 3GPP LTE usesorthogonal frequency division multiple access (OFDMA) in a downlink, anduses single carrier-frequency division multiple access (SC-FDMA) in anuplink. The 3GPP LTE employs multiple input multiple output (MIMO)having up to four antennas. In recent years, there is an ongoingdiscussion on 3GPP LTE-advanced (LTE-A) that is an evolution of the 3GPPLTE.

The 3GPP LTE-A employs various techniques such as carrier aggregation,relay, etc. The 3GPP LTE system is a single carrier system supportingonly one bandwidth (i.e., one component carrier) among {1.4, 3, 5, 10,15, 20} MHz. On the other hand, the LTE-A employs multiple carriersusing carrier aggregation. The component carrier is defined with acenter frequency and a bandwidth. The component carrier may correspondto one cell. A multiple carrier system uses a plurality of componentcarriers having a narrower bandwidth than a full bandwidth.

To decrease interference caused by uplink transmission between userequipments (UEs), it is important for a base station (BS) to maintainuplink time alignment of the UEs. The UE may be located in any area in acell. An uplink signal transmitted by the UE may arrive to the BS at adifferent time according to the location of the UE. A signal arrivaltime of a UE located in a cell edge is longer than a signal arrival timeof a UE located in a cell center. On the contrary, the signal arrivaltime of the UE located in the cell center is shorter than the signalarrival time of the UE located in the cell edge.

To decrease interference between the UEs, the BS needs to performingscheduling so that uplink signals transmitted by the UEs in the cell canbe received within a boundary every time. The BS has to properlyregulate transmission timing of each UE according to a situation of eachUE. Such a regulation is called maintenance of time alignment.

As multiple carriers are introduced, uplink time alignment needs to bemaintained in each component carrier (or serving cell). A signalingoverhead for maintaining the uplink time alignment may increase inproportion to the number of component carriers.

Accordingly, there is a need for a method of maintaining uplink timealignment for a plurality of component carriers.

DISCLOSURE Technical Problem

The present invention provides a method and apparatus for receiving atiming alignment command for a plurality of serving cells.

The present invention also provides a method and apparatus fortransmitting a timing alignment command for a plurality of servingcells.

Technical Solution

In an aspect, a. method of receiving a timing advance command by a userequipment in a wireless communication system is provided. The methodincludes receiving information regarding a time alignment group from abase station, and receiving the timing advance command corresponding tothe time alignment group from the base station.

The method may further includes applying the timing advance command toat least one serving cell belonging to the time alignment group.

The information regarding the time alignment group may includeinformation regarding a time alignment group identifier.

The timing advance command may be received together with the timealignment group identifier.

The time alignment group identifier may be a logical channel identifier.

The time alignment group identifier may be included in a medium accesscontrol (MAC) control element (CE).

The time alignment group identifier may be included in a MAC sub-header.

The time alignment group may be determined based on a serving cell inwhich the timing advance command is received.

The step of applying of the timing advance command to the at least oneserving cell belonging to the time alignment group may includerestarting an uplink alignment timer for the time alignment group.

The timing advance command may be for uplink time alignment.

In another aspect, a method of transmitting a timing advance command bya base station in a wireless communication system is provided. Themethod includes transmitting information regarding a time alignmentgroup to a user equipment, and transmitting the timing advance commandcorresponding to the time alignment group to the user equipment.

Advantageous Effects

A plurality of serving cells are grouped, and the same uplink timealignment value is applied to each group. A signaling overhead formaintaining uplink time alignment between a base station and a userequipment can be decreased.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

FIG. 2 is a diagram illustrating a radio protocol architecture for auser plane.

FIG. 3 is a diagram illustrating a radio protocol architecture for acontrol plane.

FIG. 4 shows an example of multiple carriers.

FIG. 5 shows a second-layer structure of a base station for multiplecarriers.

FIG. 6 shows a second-layer structure of a user equipment for multiplecarriers.

FIG. 7 shows a structure of a medium access control (MAC) protocol dataunit (PDU) in 3rd generation partnership project (3GPP) long termevolution (LTE).

FIG. 8 shows various examples of a MAC sub-header.

FIG. 9 shows an example of a MAC control element (CE) including a timealignment group identifier.

FIG. 10 shows an example of a time alignment group identifier using abitmap.

FIG. 11 is a flowchart showing a method of maintaining uplink timealignment according to an embodiment of the present invention.

FIG. 12 is a block diagram showing a wireless communication system forimplementing an embodiment of the present invention.

MODE FOR INVENTION

FIG. 1 shows a wireless communication system to which the presentinvention is applied. The wireless communication system may also bereferred to as an evolved-UMTS terrestrial radio access network(E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides acontrol plane and a user plane to a user equipment (UE) 10. The UE 10may be fixed or mobile, and may be referred to as another terminology,such as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), a mobile terminal (MT), a wireless device, etc. The BS 20is generally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC) 30, more specifically, to a mobility management entity (MME)through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway(P-GW). The MME has access information of the UE or capabilityinformation of the UE, and such information is generally used formobility management of the UE. The S-GW is a gateway having an E-UTRANas an end point. The P-GW is a gateway having a PDN as an end point.

A radio interface between the UE and the BS is called a Uu interface.Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. Among them, a physical (PHY) layer belonging to the first layerprovides an information transfer service by using a physical channel,and a radio resource control (RRC) layer belonging to the third layerserves to control a radio resource between the UE and the network. Forthis, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram illustrating a radio protocol architecture for auser plane. FIG. 3 is a diagram illustrating a radio protocolarchitecture for a control plane. The user plane is a protocol stack foruser data transmission. The control plane is a protocol stack forcontrol signal transmission.

Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with aninformation transfer service through a physical channel. The PHY layeris connected to a medium access control (AC) layer which is an upperlayer of the PHY layer through a transport channel. Data is transferredbetween the MAC layer and the PHY layer through the transport channel.The transport channel is classified according to how and with whatcharacteristics data is transmitted through a radio interface.

Between different PHY layers, i.e., a PHY layer of a transmitter and aPHY layer of a receiver, data are transferred through the physicalchannel. The physical channel is modulated using an orthogonal frequencydivision multiplexing (OFDM) scheme, and utilizes time and frequency asa radio resource.

A function of the MAC layer includes mapping between a logical channeland a transport channel and multiplexing/de-multiplexing on a transportblock provided to a physical channel over a transport channel of a MACservice data unit (SDU) belonging to the logical channel. The MAC layerprovides a service to a radio link control (RLC) layer through thelogical channel.

A function of the RLC layer includes RLC SDU concatenation,segmentation, and reassembly. To ensure a variety of quality of service(QoS) required by a radio bearer (RB), the RLC layer provides threeoperation modes, i.e., a transparent mode (TM), an unacknowledged mode(UM), and an acknowledged mode (AM). The AM RLC provides errorcorrection by using an automatic repeat request (ARQ).

Functions of a packet data convergence protocol (PDCP) layer in the userplane include user data delivery, header compression, and ciphering.Functions of a PDCP layer in the control plane include control-planedata delivery and ciphering/integrity protection.

A radio resource control (RRC) layer is defined only in the controlplane. The RRC layer serves to control the logical channel, thetransport channel, and the physical channel in association withconfiguration, reconfiguration and release of radio bearers (RBs).

An RB is a logical path provided by the first layer (i.e., the PHYlayer) and the second layer (i.e., the MAC layer, the RLC layer, and thePDCP layer) for data delivery between the UE and the network. Theconfiguration of the RB implies a process for specifying a radioprotocol layer and channel properties to provide a particular serviceand for determining respective detailed parameters and operations. TheRB can be classified into two types, i.e., a signaling RB (SRB) and adata RB (DRB). The SRB is used as a path for transmitting an RRC messagein the control plane. The DRB is used as a path for transmitting userdata in the user plane.

When an RRC connection is established between an RRC layer of the UE andan RRC layer of the network, the UE is in an RRC connected state, andotherwise the UE is in an RRC idle state.

Data is transmitted from the network to the UE through a downlinktransport channel. Examples of the downlink transport channel include abroadcast channel (BCH) for transmitting system information and adownlink-shared channel (SCH) for transmitting user traffic or controlmessages. The user traffic of downlink multicast or broadcast servicesor the control messages can be transmitted on the downlink-SCH or anadditional downlink multicast channel (MCH). Data are transmitted fromthe UE to the network through an uplink transport channel. Examples ofthe uplink transport channel include a random access Channel (RACH) fortransmitting an initial control message and an uplink SCH fortransmitting user traffic or control messages.

Examples of logical channels belonging to a higher channel of thetransport channel and mapped onto the transport channels include abroadcast channel (BCCH), a paging control channel (PCCH), a commoncontrol channel (CCCH), a multicast control channel (MCCH), a multicasttraffic channel (MTCH), etc.

Now, maintenance of uplink time alignment of 3GPP LTE will be described.

To decrease interference caused by uplink transmission between UEs, itis important for a BS to maintain uplink time alignment of the UEs. TheUE may be located in any area in a cell. An uplink signal transmitted bythe UE may arrive to the BS at a different time according to thelocation of the UE. A signal arrival time of a UE located in a cell edgeis longer than a signal arrival time of a UE located in a cell center.On the contrary, the signal arrival time of the UE located in the cellcenter is shorter than the signal arrival time of the UE located in thecell edge.

To decrease interference between the UEs, the BS needs to performingscheduling so that uplink signals transmitted by the UEs in the cell canbe received within a boundary every time. The BS has to properlyregulate transmission timing of each UE according to a situation of eachUE. Such a regulation is called maintenance of time alignment.

A random access procedure is one of methods for managing time alignment.The UE transmits a random access preamble to the BS. The BS calculates atime alignment value for advancing or delaying transmission timing ofthe UE on the basis of the received random access preamble. In addition,the BS transmits a random access response including the calculated timealignment value to the UE. The UE updates the transmission timing byusing the time alignment value.

In another method, the BS receives a sounding reference signal from theUE periodically or randomly, calculates the time alignment value of theUE by using the sounding reference signal, and reports a MAC controlelement (CE) to the UE.

The time alignment value sent by the BS to the UE to maintain uplinktime alignment is called a timing alignment command or a timing advancecommand.

Since the UE has mobility in general, the transmission timing of the UEvaries depending on a moving speed, a location, or the like of the UE.Therefore, the time alignment value received by the UE is preferablyvalid during a specific time period. For this, a time alignment timer isused.

When time alignment is updated after receiving the time alignment valuefrom the BS, the UE starts or restarts the time alignment timer. The UEcan perform uplink transmission only when the time alignment timer isrunning. A value of the time alignment timer may be reported by the BSto the UE by using system information or an RRC message such as a radiobearer reconfiguration message.

When the time alignment timer expires or when the time alignment timerdoes not run, the UE does not transmit any uplink signal except for therandom access preamble under the assumption that time alignment is notachieved between the BS and the UE.

FIG. 7 shows a structure of a MAC PDU in 3GPP LTE.

The MAC PDU includes a MAC header, a MAC control element (CE), and atleast one MAC service data unit (SDU). The MAC header includes at leastone sub-header. Each sub-header corresponds to the MAC CE and the MACSDU. The sub-header has the same length and property as the MAC CE andthe MAC SDU. The MAC SDU is a data block provided from a higher layer(e.g., an RLC layer or an RRC layer) of a MAC layer. The MAC CE is usedto deliver control information of the MAC layer such as a buffer statusreport.

FIG. 8 shows various examples of a MAC sub-header.

Descriptions on each field are as follows.

-   -   R (1 bit): A reserved field.    -   E (1bit): An extended field. It indicates whether there are F        and L fields in a next field.    -   LCID (5 bit): A logical channel ID field. It indicates a type of        the MAC CE or a specific logical channel to which the MAC SDU        belongs.    -   F (1 bit): A format field. It indicates whether a next L field        has a size of 7 bits or 15 bits.    -   L (7 or 15 bit): A length field. It indicates a length of the        MAC CE or MAC SDU corresponding to the MAC sub-header.

The F and L fields are not included in a MAC sub-header corresponding toa fixed-sized MAC CE.

(A) and (B) of FIG. 8 show exemplary structures of a MAC sub-headercorresponding to a variable-sized MAC CE and MAC SDU. (C) of FIG. 8shows an exemplary structure of a MAC sub-header corresponding to afixed-sized MAC CE.

Now, a multiple carrier system will be described.

A 3GPP LTE system supports a case where a downlink bandwidth and anuplink bandwidth are set differently under the premise that onecomponent carrier (CC) is used. The CC is defined with a centerfrequency and a bandwidth. This implies that the 3GPP LTE is supportedonly when the downlink bandwidth and the uplink bandwidth are identicalor different in a situation where one CC is defined for each of adownlink and an uplink. For example, the 3GPP LTE system supports up to20 MHz and the uplink bandwidth and the downlink bandwidth may bedifferent from each other, but supports only one CC in the uplink andthe downlink.

Spectrum aggregation (or bandwidth aggregation, also referred to ascarrier aggregation) supports a plurality of CCs. The spectrumaggregation is introduced to support an increasing throughput, toprevent a cost increase caused by using a broadband radio frequency (RF)element, and to ensure compatibility with legacy systems.

FIG. 4 shows an example of multiple carriers. There are five CCs, i.e.,CC #1, CC #2, CC #3, CC #4, and CC #5, each of which has a. bandwidth of20 MHz. Therefore, if the five CCs are allocated in a granularity of aCC unit having the bandwidth of 20 MHz, a bandwidth of up to 100 MHz canbe supported.

The bandwidth of the CC or the number of the CCs are exemplary purposesonly. Each CC may have a different bandwidth. The number of downlink CCsand the number of uplink CCs may be identical to or different from eachother.

FIG. 5 shows a second-layer structure of a BS for multiple carriers.FIG. 6 shows a second-layer structure of a UE for multiple carriers.

A MAC layer can manage one or more CCs. One MAC layer includes one ormore HARQ entities. One HARQ entity performs HARQ on one CC. Each HARQentity independently processes a transport block on a transport channel.Therefore, a plurality of HARQ entities can transmit or receive aplurality of transport blocks through a plurality of CCs.

One CC (or a CC pair of a downlink CC and an uplink CC) may correspondto one cell. When a synchronous signal and system information areprovided by using each downlink CC, it can be said that each downlink CCcorresponds to one serving cell. When the UE receives a service by usinga plurality of downlink CCs, it can be said that the UE receives theservice from a plurality of serving cells.

The BS can provide the plurality of serving cells to the UE by using theplurality of downlink CCs. Accordingly, the UE and the BS cancommunicate with each other by using the plurality of serving cells.

Since the plurality of serving cells have different center frequencies,uplink time alignment needs to be regulated for each serving cell.

However, an overhead caused by signaling for transmission of a timealignment value may increase in proportion to the number of servingcells. If the time alignment value for one serving cell can betransmitted using one MAC CE similarly to the conventional 3GPP LTE, itis required to transmit 4 MAC CEs to maintain the uplink time alignmentfor 4 serving cells.

According to a frequency property of the serving cells, there may beserving cells having similar time alignment properties. The similar timealignment property implies that the serving cells have similar timealignment values. For example, serving cells using contiguous frequencybands may have the similar time alignment properties.

A method for optimizing signaling for uplink time alignment regulationbetween serving cells having similarly time alignment properties isproposed.

According to the present invention, a method of grouping serving cellsby using similarities in the change of time alignment and managing atiming alignment command for each group will be proposed.

Management of the timing alignment command proposed for each group canbe allowed if carrier aggregation is possible.

Management of the timing alignment command proposed for each group canbe allowed if a plurality of time alignment values are required.

Grouping of the serving cells may be performed by the BS. The BS mayreport the grouping result to the UE. The BS determines a specific timealignment group to which each serving cell is included, and reportsgroup information to the UE. The group information may be transmitted tothe UE by using an RRC message, a MAC message, and/or a PDCCH message.

Each of the UE and the BS may perform grouping by using a pre-definedmethod. Information related to the pre-defined method may be transmittedby the BS to the UE by using the RRC message, the MAC message, and/orthe PDCCH message.

The pre-defined method may be based on a frequency band of each servingcell. It is defined that serving cells using a frequency band includedin a specific frequency band belong to the same time alignment group.For example, it is assumed that frequencies X1 and X2 are included in afrequency band X, and frequencies Y1 and Y2 are included in a frequencyband Y. Further, it is also assumed that a 1st serving cell using thefrequency X1 and a 2nd serving cell using the frequency Y1 are assignedto the UE. If the BS additionally assigns a 3rd serving cell which usesthe frequency Y2 to the UE, the UE and the BS may determine that the 3rdserving cell is included in the same time alignment group as the 2ndserving cell.

The time alignment group includes one or more serving cells (or CCs)which use the same time alignment value to maintain uplink timealignment.

A time alignment value for a specific time alignment group is valid onlyfor serving cells of the time alignment group.

The time alignment group can be identified by using a time alignmentgroup identifier (ID). The BS may allocate the time alignment group IDto which the serving cell belongs, when assigning the serving cell tothe UE. The UE and the BS determine that serving cells which use thesame time alignment group ID are included in the same time alignmentgroup.

The BS may transmit the time alignment group ID together when sending atime alignment value for a specific time alignment group to the UE. TheUE may apply the time alignment value to the serving cells identified bythe time alignment group ID.

The time alignment group ID can be expressed by using various methods.

In one embodiment, a logical channel ID (LCID) included in a MACsub-header may be used as the time alignment group ID for each timealignment group. As shown in FIG. 7, a MAC protocol data unit (PDU)includes a MAC header, a MAC service data unit (SDU), and a MAC CE. TheMAC header includes at least one MAC sub-header.

Time alignment groups are mapped to respective logical channel IDs. TheMAC CE including a time alignment value and the MAC sub-header includingthe LCID are included in the MAC PDU. Upon receiving the MAC PDU, the UEidentifies the time alignment group on the basis of the LCID, andapplies the time alignment value to serving cells included in the timealignment group.

In another embodiment, some of bits included in the MAC CE may be usedas the time alignment group ID. Since 2 bits are reserved in the MAC CEindicating the previous time alignment value, the reserved bits may beused as the time alignment group ID. FIG. 9 shows an example of the MACCE including the time alignment group ID.

In another embodiment, the time alignment group ID may be a bitmapindicating a time alignment value for each time alignment group.

FIG. 10 shows an example of a time alignment group ID using a bitmap.Assume that there are two time alignment groups. A MAC CE includes a 1sttime alignment value for a 1st time alignment group and a 2nd timealignment value for a 2nd time alignment group. A time alignment groupID having a bitmap ‘10’ indicates the 1st time alignment group. A timealignment group ID having a bitmap ‘01’ indicates the 2nd time alignmentgroup.

To determine a time alignment group to which the time alignment value isapplied, a UE may use information regarding a serving cell in which thetime alignment value is received. Assume that a time alignment group towhich each serving cell belongs is pre-known to both the UE and a BS.The BS transmits the time alignment value to the UE by using a servingcell belonging to a specific time alignment group. The UE applies thetime alignment value to serving cells belonging to the time alignmentgroup including the serving cell in which the time alignment value isreceived.

FIG. 11 is a flowchart showing a method of maintaining uplink timealignment according to an embodiment of the present invention.

A BS delivers time alignment group information to a UE (step S810). Thetime alignment group information may include information regarding aplurality of time alignment groups including at least one serving cell.When the serving cell is added or modified by using an RRC connectionreconfiguration message, the BS may transmits information regarding atime alignment group including the serving cell to the UE. The timealignment group information may include a time alignment group ID.

One time alignment timer runs for each time alignment group. After beingcompletely connected to the BS, the UE may apply a time alignment valueincluded in a random access response and then may start the timealignment timer.

When there is a need to regulate uplink time alignment of a specificserving cell or serving cells belonging to a specific time alignmentgroup, the BS transmits the time alignment value to the UE (step S820).While the time alignment timer is running, the time alignment value maybe transmitted by being included in a MAC PDU and/or the random accessresponse. The time alignment group ID may be transmitted to the UE inaddition to the time alignment value.

The UE applies the time alignment value to serving cells belonging tothe time alignment group (step S830). The UE may use the aforementionedmethod of determining the time alignment group to determine a timealignment group to which the time alignment value is applied. The timealignment group can be identified by the time alignment group ID.Alternatively, the time alignment group can be determined based on aserving cell in which the time alignment value is transmitted.

When the time alignment value is applied, the UE starts or restarts atime alignment timer of the time alignment group. At the expiry of thetime alignment timer, the UE may inactivate serving cells in the timealignment group or may release an uplink resource. The UE does nottransmit any other uplink signals except for a random access preamble toinactive serving cells until receiving a command for activating theinactive serving cells from the BS.

FIG. 12 is a block diagram showing a wireless communication system forimplementing an embodiment of the present invention.

A BS 50 includes a processor 51, a memory 52, and a radio frequency (RF)unit 53. The memory 52 is coupled to the processor 51, and stores avariety of information for driving the processor 51. The RF unit 53 iscoupled to the processor 51, and transmits and/or receives a radiosignal. The processor 51 implements the proposed functions, procedures,and/or methods. In the embodiment of FIG. 11, the operation of the BS 50can be implemented by the processor 51.

A UE 60 includes a processor 61, a memory 62, and an RF unit 63. Thememory 62 is coupled to the processor 61, and stores a variety ofinformation for driving the processor 61. The RF unit 63 is coupled tothe processor 61, and transmits and/or receives a radio signal. Theprocessor 61 implements the proposed functions, procedures, and/ormethods. In the embodiment of FIG. 11, the operation of the UE 60 can beimplemented by the processor 61.

The processor may include application-specific integrated circuit(ASIC), other chipset, logic circuit and/or data processing device. Thememory may include read-only memory (ROM), random access memory (RAM),flash memory, memory card, storage medium and/or other storage device.The RF unit may include baseband circuitry to process radio frequencysignals. When the embodiments are implemented in software, thetechniques described herein can be implemented with modules (e.g.,procedures, functions, and so on) that perform the functions describedherein. The modules can be stored in memory and executed by processor.The memory can be implemented within the processor or external to theprocessor in which case those can be communicatively coupled to theprocessor via various means as is known in the art.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

The invention claimed is:
 1. A method of receiving a timing advancecommand by a user equipment in a wireless communication system, themethod comprising: receiving, from a base station, the timing advancecommand corresponding to a time alignment group; and applying thereceived timing advance command to at least one serving cell belongingto the time alignment group, wherein the timing advance command isreceived together with a time alignment group identifier, wherein thetime alignment group identifier identifies the time alignment group towhich the timing advance command is applied, wherein the time alignmentgroup identifier is included in a medium access control (MAC) controlelement (CE), and wherein the MAC CE includes both the time alignmentgroup identifier and the timing advance command.
 2. The method of claim1, wherein the time alignment group identifier is a time advance groupidentifier or a logical channel identifier.
 3. The method of claim 1,wherein the applying of the received timing advance command to the atleast one serving cell belonging to the time alignment group comprisesrestarting an uplink alignment timer for the time alignment group. 4.The method of claim 1, wherein the timing advance command is used for anuplink time alignment.
 5. A method of transmitting a timing advancecommand by a base station in a wireless communication system, the methodcomprising: transmitting, to a user equipment, the timing advancecommand corresponding to a time alignment group, wherein the timingadvance command is transmitted together with a time alignment groupidentifier, wherein the time alignment group identifier identifies thetime alignment group to which the timing advance command is applied,wherein the time alignment group identifier is included in a mediumaccess control (MAC) control element (CE), and wherein the MAC CEincludes both the time alignment group identifier and the timing advancecommand.
 6. The method of claim 5, wherein the time alignment groupidentifier is a time advance group identifier or a logical channelidentifier.
 7. The method of claim 5, wherein the time alignment groupis determined based on a serving cell in which the timing advancecommand is transmitted.
 8. The method of claim 5, wherein the timingadvance command is for uplink time alignment.
 9. A User Equipment (UE)for receiving a timing advance command in a wireless communicationsystem, the UE comprising: a transceiver configured to transmit orreceive a data; a memory configured to store the data; and a processorconfigured to cooperate with the transceiver and the memory to: receivethe timing advance command corresponding to the time alignment groupfrom a base station; and apply the received timing advance command to atleast one serving cell belonging to the time alignment group, whereinthe timing advance command is received together with a time alignmentgroup identifier, wherein the time alignment group identifier identifiesthe time alignment group to which the timing advance command is applied,wherein the time alignment group identifier is included in a mediumaccess control (MAC) control element (CE), and wherein the MAC CEincludes both the time alignment group identifier and the timing advancecommand.
 10. The UE of claim 9, wherein the time alignment groupidentifier is a time advance group identifier or a logical channelidentifier.
 11. The UE of claim 9, wherein the applying of the receivedtiming advance command to the at least one serving cell belonging to thetime alignment group comprises restarting an uplink alignment timer forthe time alignment group.
 12. The UE of claim 9, wherein the timingadvance command is used for an uplink time alignment.
 13. A base station(BS) for transmitting a timing advance command in a wirelesscommunication system, the BS comprising: a transceiver configured totransmit or receive a data; a memory configured to store the data; and aprocessor configured to cooperate with the transceiver and the memoryto: transmit the timing advance command corresponding to a timealignment group to a user equipment, wherein the timing advance commandis transmitted together with a time alignment group identifier, whereinthe time alignment group identifier identifies the time alignment groupto which the timing advance command is applied, wherein the timealignment group identifier is included in a medium access control (MAC)control element (CE), and wherein the MAC CE includes both the timealignment group identifier and the timing advance command.
 14. The BS ofclaim 13, wherein the time alignment group identifier is a time advancegroup identifier or a logical channel identifier.
 15. The BS of claim13, wherein the time alignment group is determined based on a servingcell in which the timing advance command is transmitted.
 16. The BS ofclaim 13, wherein the timing advance command is for uplink timealignment.